Gypsum pond water, or "pond water" as it will hereinafter be referred to, originates when by-product gypsum from the attack system of wet process phosphoric acid plants is slurried with water and pumped to large ponds where the gypsum settles out. The remaining supernatant water is highly acidic and contains various dissolved impurities, some of which are environmentally undesirable and cannot be discharged into water ways. Because of the acidity and the presence of the impurities, governmental regulations prohibit the discharge of the pond water unless it is treated to neutralize the acidity and to reduce the concentration of the undesirable dissolved impurities to acceptable levels.
In much of the southeastern U.S. where annual rainfall exceeds evaporation, discharge of pond water is often necessary and phosphoric acid plants in the area are consequently obliged to frequently treat pond water for discharge. The typical neutralization treatment consists of a two-stage liming operation using lime/limestone to elevate the pH and to precipitate the impurities as insoluble calcium compounds and silica. However, the costs of such a treatment system are high. Furthermore, substantial phosphate values are lost in the precipitated sludge. Wet process plant operators, therefore, try to minimize treatment costs by consuming as much contaminated pond water as possible in phosphoric acid production to remain in water balance.
A major potential internal use for the pond water exists in plants which wet-grind phosphate rock in ball mills. These plants often use pond water instead of fresh water in the grinding circuit. The pond water is typically neutralized to reduce the corrosion rate of the grinding equipment. Several U.S. patents describe neutralization processes using various chemicals and procedures. See, for example, U.S. Pat. No. 4,402,923 to Lang. The Lang process partially neutralizes pond water with ammonia. The Lang process is not completely satisfactory because the treated water is still somewhat corrosive and, additionally, the ammonium ion can cause precipitation problems in subsequent phosphoric acid processing operations.
Most pond water treatment processes use lime/limestone to neutralize the water. U.S. Pat. No. 4,320,012 to Palm et al describes a two-stage neutralization method for pond water wherein lime/limestone is used to raise the pH of the pond water to about 4.0 in the first stage and to about 8.0-11.0 in the second stage. The flocculent precipitate produced in the second neutralization stage is disposed of by dissolving it in incoming untreated pond water prior to the first neutralization stage. The process, while an improvement over some of the prior art pond water neutralization processes, still requires two liming steps. Also, the neutralization is carried out at such a high pH, that excessive amounts of lime/limestone are required. Additionally, most phosphate values originally contained in the pond water report in the first stage settler underflow and are thus lost in a stream destined for disposal.
In U.S. Pat. No. 4,472,368 to O'Neill et. al., pond water is treated using a partial liming process. The process removes nearly all of the fluoride component, giving a treated water which can be used in the phosphate rock wet grinding circuit without causing excessive corrosion. From 55% to 70% of the phosphate component remains in solution and is thereby recovered. However, only that amount of water required for phosphate rock grinding may be consumed in this way. No excess water suitable for discharge is produced.
There exists a need in the wet process phosphoric acid industry for improved pond water and waste water treating processes which minimize the consumption of lime/limestone and which precipitate the bulk of the undesirable fluoride and fluosilicic compounds while retaining much of the phosphate values in a liquid stream for subsequent recovery in phosphoric acid production.
The present invention is directed to an improved process for treating pond water and waste water wherein the consumption of lime is minimized and which allows for the recovery of much of the phosphate values while precipitating and disposing of most of the fluoride and fluosilicic compounds. The process of the invention utilizes a two-stage process for treating pond water wherein lime addition is needed in only one of the stages thereby resulting in reduced lime consumption compared to conventional waste water treating processes.
The process of the invention further provides a process for treating pond water wherein a substantial fraction of its P.sub.2 O.sub.5 content can be concentrated into a smaller stream of P.sub.2 O.sub.5 -enriched effluent suitable for use in the wet grinding circuit of a phosphoric acid plant and wherein the balance of the aqueous effluent is virtually void of phosphate and fluoride ions and meets environmental standards for discharge.